The human infant is typically sterile in utero and then born into a world dominated by microbes, which selectively assume residence of the body surfaces and enteric tract of the newborn. The assembly of the early human microbiome is particularly an issue relevant to the very low birth weight (VLBW; <1500 g) infant, where infection and activation of immunological and inflammatory responses produce acute morbidity and mortality and exacerbate co-morbid diseases of the eyes, enteric tract, and central nervous system. However, the selective forces that alter primary colonization and secondary persistence of more or less pathogenic microbes within the developing microbiota are not well understood, particularly in humans. Specifically, the extent to which colonization is recognized by the adaptive immune response and how the adaptive responses contribute to shaping the microbiome of the premature infant, and how the microbiome influences concurrent and subsequent adaptive immune development, is poorly understood. The immediate goal of the proposed research is to identify antigens from the pioneering microbiome of the intestine in the preterm infant that are recognized by natural birth antibodies and the adaptive humoral response. This work will then determine whether and how the peripheral and mucosal antibody response alters the dynamics of secondary colonization. The central hypothesis is that natural antibodies present at birth modify primary colonization while antigens of pioneering Gram negative enteric bacteria drive adaptive antibody responses that alter secondary colonization and long-term immune memory. The hypothesis will be addressed through an innovative combination of molecular tools in the following aims: 1) To determine the relationship between natural and adaptive antibodies and colonization of the VLBW infant intestinal tract, 2) To determine the specificity of breast milk IgA to the breast milk microbiome and the early intestinal microbiome in the VLBW infant, and 3) To measure the specific antigens of the neonatal microbiome recognized by antibodies in the first year of life. These studies will uniquely define the constellation of antigens recognized by preterm infant and maternal natural and adaptive antibodies as the infant becomes colonized. By determining the dynamic interface between mucosal antibody response and specific antigens recognized by the newborn human host, new interventions may be envisioned through which to alter primary and secondary colonization of infants through stimulation of natural and adaptive antibodies. In turn these advents may lead to approaches to alter long-term outcomes including infectious diseases, allergy, and metabolic syndrome.